Scanners are widely used to process image of objects into digital forms. Depending on the property of the scanned object, different types of scanners are used. Generally, a transmissive scanner is used for scanning transparent objects, and a reflective scanner is used for scanning opaque objects. Examples of opaque objects include paper sheets, photographs, etc., and common transparent objects are for example slides and films. For scanning both opaque and transparent objects with the same scanner, a dual-mode scanner for optional transmissive/reflective scanning was developed.
Please refer to FIG. 1, which is a schematic diagram viewing from a side of a conventional dual-mode image scanner. In the image scanner, both reflective light source 12 and penetrative light source 13 are provided in the lower housing 101 and the upper housing 102, respectively, and selectively turned on for different scanning purposes. For reflective-type scanning operation, it is the reflective light source 12 disposed in the carriage 11 turned on while the penetrative light source 13 is turned off. The light emitted by the light source 12 is reflected by the scanned object 14, which is clamped between the upper housing 102 and the lower housing 101. The reflective image then enters the carriage 11, guided by a mirror set 15, focused by a lens set 16 and processed by an image pickup device such as a charge coupled device (CCD) 17 to complete scanning. On the other hand, for penetrative-type scanning operation, it is the penetrative light source 13 disposed opposite to the carriage 11 turned on while the reflective light source 12 is turned off. The light emitted by the light source 13 penetrates through the scanned object 14 and then enters the carriage 11. The image is also processed by the elements 15, 16 and 17 in the carriage 11 to generate electric signals in response to the image of the scanned object 14.
Nowadays, the penetrative light source 13 is implemented with a planar light source so that the light source 13 does not have to move with the carriage 11 to scan lines. The planar light source 13 generally includes a light-emitting element 21 for emitting light and a light-guiding plate 20 for diffusing light all over the plate, as shown in FIG. 2A, which is advantageous for small thickness and high conformity to the upper housing. The light-emitting element 21 can be, for example, a linear cold cathode lamp. Since the light-emitting element 21 is disposed at one side of the light-guiding plate 20 that afterwards diffuses light to the opposite side, the light intensity at both ends of each scan line may differ a lot. Consequently, the resulting image quality would be unsatisfactory. For solving this problem, double lamps 22 and 23 are disposed at both sides of the light-guiding plate 20 to unify the light intensity provided for the scanned object. Since two lamps are used, it is apparent that cost and power consumption increase.